The present invention relates to a circuit board connection structure, a method for forming the circuit board connection structure, a display device having the circuit board connection structure, and a method for fabricating the display device.
FIG. 8 is a cross-sectional view of a conventional active matrix liquid crystal display device. The active matrix liquid crystal display device of FIG. 8 includes a liquid crystal panel 2 and a circuit board 6 connected to each other via a flexible printed wiring board 3. The liquid crystal panel 2 includes an active matrix substrate 2A, a counter substrate 2B and a liquid crystal layer 1 interposed between the substrates 2A and 2B. A driving LSI is mounted on the flexible printed wiring board 3. A chip capacitor, a control IC and the like are mounted on the circuit board 6.
To drive the liquid crystal layer 1, an input signal must be supplied to the active matrix substrate 2A. In the liquid crystal display device of FIG. 8, the flexible printed wiring board 3 and the circuit board 6 are electrically connected to the active matrix substrate 2A, to thereby allow supply of a predetermined input signal from the flexible printed wiring board 3 and the circuit board 6 to the active matrix substrate 2A. The conventional structures of the circuit board 6 and the flexible printed wiring board 3, as well as the connection structure of these boards, will be described with reference to FIGS. 9 and 10.
FIG. 9 is an illustration of a conductive adhesive used for bonding between the circuit board 6 and the flexible printed wiring board 3. FIG. 10 is a partial plan view of the circuit board 6 and the flexible printed wiring board 3 of the liquid crystal display device of FIG. 8, viewed in the direction of the arrow in FIG. 8.
An anisotropic conductive film (ACF), a conductive adhesive tape shown in FIG. 9, is used for bonding between the circuit board 6 and the flexible printed wiring board 3. The ACF shown in FIG. 9 has a double-layer structure composed of an ACF layer 4 as a conductive resin layer (conductive adhesive layer) and a separator 5. The separator 5 of the ACF is finally removed and the remaining ACF layer 4 is set under heat to thereby bond the two boards 6 and 3 together.
Referring to FIG. 10, the circuit board 6 has a plurality of stripe terminal electrodes 7 and 8 formed in an edge region thereof. The flexible printed wiring board 3 also has a plurality of stripe terminal electrodes 20 and 22 in an edge region thereof. The circuit board 6 and the flexible printed wiring board 3 are connected with each other in the following manner.
First, the ACF shown in FIG. 9 is placed on the circuit board 6 to cover the terminal electrodes 7 and 8 partly and subjected to temporary press bonding to the circuit board 6 (including heating). The separator 5 is then separated from the ACF layer 4. Thereafter, the flexible printed wiring board 3 is aligned with the circuit board 6, placed on the circuit board 6 via the ACF layer 4, and subjected to final press bonding (including heating). In this way, the circuit board 6 and the flexible printed wiring board 3 are bonded together via the ACF layer 4, and this electrically connects the terminal electrodes 7 and 8 of the circuit board 6 with the terminal electrodes 20 and 22 of the flexible printed wiring board 3.
In the conventional circuit board 6, the terminal electrodes 7, which are to be electrically connected with the outermost terminal electrodes 20 located closest to the edges of the flexible printed wiring board 3, among the plurality of stripe terminal electrodes 7 and 8, have a shape wider than the terminal electrodes 8 and have a continuous surface. The surface of the terminal electrodes 7 is made continuous to facilitate detection of whether or not the ACF layer 4 has separated from the terminal electrodes after the temporary press bonding of the ACF layer 4 to the circuit board 6. The terminal electrodes 7 are made wide because faulty connection will occur if the terminal electrodes of the flexible printed wiring board 3 fail to match with the terminal electrodes 7. The reason why only the terminal electrodes 7 are made wide selectively is as follows.
The ACF layer 4 is set under heat during the bonding between the circuit board 6 and the flexible printed wiring board 3. During cooling of these boards 6 and 3 after the heating for the setting, the flexible printed wiring board 3 is generally cooled more easily than the circuit board 6. Therefore, portions of the ACF layer 4 existing in regions corresponding to the edges of the flexible printed wiring board 3 tend to separate, and as a result, faulty electrical connection tends to occur between the ACF layer 4 and the terminal electrodes of the flexible printed wiring board 3 particularly in these regions. To suppress this occurrence of faulty electrical connection, the terminal electrodes 7 in the regions corresponding to the edges of the flexible printed wiring board 3 are made wide.
However, the conventional circuit board connection structure shown in FIG. 10 has the following problems.
First, when the conductive resin is heated (during temporary press bonding or final press bonding), the conductive resin on the terminal electrode 7 tends to flow from the surface thereof down to the board due to the solid shape of the terminal electrode 7. As a result, the conductive resin layer formed on the terminal electrode 7 becomes too thin to permit bonding of the terminal electrode 7 to the corresponding terminal electrode 20 (transfer failure) in some cases. The proportion of the bonding failure is particularly high when the terminal electrode 7 has the wide shape and an end of the conductive resin layer 4 is located on the terminal electrode 7. This causes faulty electrical connection between the boards, and thus lowers the device fabrication efficiency.
Secondly, during the bonding of the flexible printed wiring board 3 to the circuit board 6, the positioning is performed only in the direction of the pitch of the terminal electrodes 7 and 20 shown by arrow P1 in FIG. 10. No means is provided for examining a displacement in the direction of the length of the terminal electrodes 7 and 20 shown by arrow P2.
In view of the above, an object of the present invention is preventing the conductive resin layer from separating from the terminal electrodes or the circuit board. Another object of the present invention is achieving high-precision electrical connection between the terminal electrodes of the two boards with no displacement from each other.